Update glpk to 4.9
A MIP presolver were implemented (currently incomplete). It is
used internally in the routine lpx_intopt (see below).
An advanced branch-and-bound solver (the routine lpx_intopt)
were implemented.
The routine lpx_check_int to check MIP feasibility conditions
was added.
The routine lpx_print_mip was changed to print MIP feasibility
conditions.
The built-in functions sin, cos, atan, and atan2 were added to
the MathProg language.
Some typos were fixed.
Thanks to Minh Ha Duong <haduong@centre-cired.fr> (CIRED, CNRS).